Selenium is a non-metallic trace element of the sulfur group that is essential in the human diet in small concentrations. However, high concentrations of selenium have accumulated in the top soil in many areas as a result of intense irrigation agriculture. Seleniferous soils have been identified throughout California and the world in areas which are often associated with the saline drainage waters. For example, selenium contamination in the Kesterson Reservoir in California has been attributed to irrigation of nearby farms, and has reached such proportions that it has killed hundreds of water fowl and poisoned livestock. Many creatures which survived have suffered reproductive failure and birth defects as a result of selenium contamination.
Methods to detoxify selenium contaminated soils are presently limited to gathering the contaminated soil and burying it in costly, impervious plastic liners. Often high selenium concentrations are found in ponds or reservoirs, and environmental officials presently are planning to dry the ponds, scrape away the top foot of contaminated mud, and remove it to an isolated land fill.
Evaporation ponds have been widely used in the San Joaquin Valley of California for disposal of agricultural subsurface drainage water. However, selenium concentrations in some of these ponds have now reached hazardous levels in excess of 1000 .mu.g/L, which is defined in Title 22 of the California Administrative Code as hazardous waste for selenium. Hazardous wastes, according to Subchapter 15 of Title 23, must be disposed in double-lined ponds with leachate collector systems. These ponds are very expensive with reported costs as high as $200,000 per acre to construct. Since selenium is a powerful poison in small amounts, and causes gross birth deformities in both fowl and mammals, it has been a desideratum to provide a safe and effective method for the detoxification of seleniferous soils rather than to merely remove these soils to designated land fills where the potential of significant damage to the environment remains.
It is known that soils naturally contain microbes which are capable of very slowly transforming non-volatile selenium compounds into volatile compounds which are relatively non-toxic and which readily dissipate in the atmosphere. These microbes are naturally occurring and may include fungi, bacteria or certain algae. This transformation is an important link in the global cycle of the element and may have maintained the non-volatile selenium compounds at safe levels in the soil prior to the intense irrigation procedures which have exceeded the cycle's ability to maintain safe selenium levels.
We have discovered a method for greatly increasing the rate of volatilization of selenium from non-volatile selenium compounds which accelerates the transformation of toxic selenium compounds into volatile products to a rate which permits the detoxification of soil or water in situ. This method comprises exposing the non-volatile selenium compound to microbes which are capable of converting the selenium in the non-volatile selenium compound to a volatile alkylselenide, while the microbe is in the presence of one or more members of the group consisting of organic carbon sources such as proteins, amino acids or carbohydrates which enhance alkylation. Preferably, the process is conducted in the further presence of metal ions which enhance alkylation, such as cobalt, zinc and nickel, the non-volatile selenium compound is exposed to the microbes, the carbon source and the metal ions in a medium, for example, soil or water, which facilitates mixing of the components and the desired reaction.
Specifically, the non-volatile selenium compound is exposed to the microbes in the presence of one or more of the members of the group consisting of pectin, straw, starch, galacturonic acid, cellobiose, glucuronic acid, glucose, albumin, casein, gluten, methionine, and ethionine; preferably along with cobalt, zinc or nickel ions.
Albumin, casein, gluten, pectin, methionine, and ethionine are employed in an aerobic soil environment alone or in combination with cobalt, zinc or nickel ions. Preferably, moisture is maintained to the soil, most preferably at field capacity, to optimize the microbial alkylation process.
Preferably pectin, straw, starch, galacturonic acid, cellobiose, glucuronic acid, glucose, albumin, casein or gluten are included in amounts which afford at least 0.01 g carbon per kilogram of soil, and most preferably from 0.01 g to 100 g C/kg soil. The methionine or ethionine are included in amounts which afford at least 0.01 mg carbon per kilogram of soil, and most preferably from 0.01 mg to 200 g C/kg soil. The cobalt, zinc or nickel ions are included in amounts which afford at least 0.01 mg metal per kilogram of soil, and most preferably from 0.01 mg to 2000 mg metal/kg soil.